Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
1999-10-15
2001-10-30
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S121000, C423S625000
Reexamination Certificate
active
06309615
ABSTRACT:
BACKGROUND OF THE INVENTION
Bauxite is the major source of aluminium containing ore used in the production of alumina. Bauxite contains hydrated forms of aluminium oxide (alumina) that occur in several different structural forms, depending upon the number of molecules of water of hydration and the crystalline form. Most commercially useful deposits of bauxite include gibbsite (alumina trihydrate) and/or boehmite (alumina monohydrate) and/or diaspore.
Alumina is extracted from bauxite by use of the Bayer process. Briefly, the Bayer process includes the steps of contacting bauxite with a hot caustic solution to dissolve alumina therefrom. If the bauxite contains mainly gibbsite, extraction of alumina from the bauxite may be conducted using a caustic solution at a temperature generally in the range of 100 to 150° C. If the bauxite contains mainly boehmite, or diaspore higher temperatures, in the order of 200 to 300° C. are generally required. For mixed bauxites containing both gibbsite and boehmite, a double digestion process may be used.
After digestion, the bauxite/caustic solution mixture is separated into a pregnant liquor containing dissolved alumina (usually in the form of sodium aluminate) and a solids residue (usually referred to as red mud). The pregnant liquor is fed to a precipitation circuit where it is cooled and seeded with solid particles of alumina trihydrate to induce precipitation of alumina trihydrate from the pregnant liquor. The resulting precipitation slurry is separated into a spent liquor stream and a solids stream. Coarse solids represent product and are transferred to a calcination stage where they are calcined to produce alumina. Fine solids are returned as seed particles to the precipitation circuit. The spent liquor is returned to the digestion step where it is contacted with further bauxite. Between the digestion and precipitation steps, there is generally one or more washing steps and the spent liquor generally must be evaporated to obtain the required caustic concentration prior to being returned to the digestion step. The Bayer process has been used commercially for about 100 years and it is well known to persons of skill in the art.
Bauxite, in addition to containing hydrated forms of alumina, includes several impurities. The main impurities are compounds of iron, titanium and silica. The compounds of iron and titanium found in bauxite generally are insoluble in caustic solutions and have little effect on the selective extraction of alumina from bauxite. These compounds report to the red mud following digestion.
The silicon compounds present in bauxite occur mainly as quartz and as hydrated double salts with alumina, such as kaolin. Quartz dissolves slowly in caustic solutions and the other forms of silica in the bauxite may dissolve rapidly in the caustic solutions used in the digestion step. Accordingly, bauxites containing significant amounts of silica have the potential to be difficult to treat.
The presence of silica in bauxite can cause at least two major problems in the digestion of bauxite, these being:
(i) dissolution and reprecipitation of silica as complex sodium alumino-silicates, thereby consuming caustic soda; and
(ii) reprecipitation of complex sodium alumino-silicates on plant surfaces, thereby causing scale build-up. This problem is especially severe when scale builds up on heat exchange surfaces.
Prior art attempts to deal with problems associated with silica in bauxite have concentrated on either suppressing silica dissolution or completing precipitation of silica in a controlled step to minimise scaling throughout the remainder of the plant. The problems of caustic soda consumption and scaling of plant surfaces are largely independent of each other—a low silica bauxite will result in low caustic soda losses but can drive significant scaling problems while a high silica bauxite will consume large quantities of caustic soda but will result in less scaling. It is for this reason that most of the prior art dealing with the impact of silica on the Bayer process deals with only one aspect of the problem. The prior art can be grouped broadly into four areas, as discussed below:
1) Predaesilication
A significant number of refineries include a so-called predesilication operation prior to digestion where the bauxite is held at a temperature of around 100° C. for 6-18 hours. The purpose of this operation is to convert a large portion of the reactive silica to sodalite type sodium alumino-silicate which will then act as seed to rapidly convert the remaining reactive silica to sodalite type sodium alumino-silicate during digestion. The conditions under which predesilication is conducted, low caustic and alumina concentration, ensure that only a very small proportion of the total reactive silica is in solution at any given time. The primary purpose of predesilication is to ensure that conversion of reactive silica to sodalite type sodium alumino-silicate is complete so that pregnant liquor from digestion contains a minimum amount of dissolved silica which in turn minimises alumina product contamination and sodium alumino-silicate scaling during subsequent reheating of spent liquor. This operation has no impact on the amount of caustic soda consumed as a result of silica reaction. Sodium alumino-silicates are usually discarded from the Bayer plant as a component of red mud. However, a separate sodium alumino-silicate precipitation step after predesilication has been proposed.
U.S. Pat. No. 3,413,087 in the name of Roberts, assigned to Reynolds Metals Company, describes a Bayer process for extracting alumina from bauxite which includes a predesilication step to dissolve silica prior to digestion. In the predesilication step, the bauxite is mixed with spent liquor or strong liquor containing make-up caustic. The quantity of caustic present in the liquor is insufficient to dissolve all of the soluble alumina in the bauxite but is sufficient to dissolve substantially all of the soluble silica in the bauxite. However, only a small fraction of the soluble silica is in solution at any time. The slurry (of bauxite and liquor) is maintained in the predesilication stage (called a predigestion stage in the patent) at a temperature of from 150° F. to the temperature used in the digestion step for a period of time (e.g. 30 minutes to 12 hours) to allow the dissolved silica to crystallize and precipitate as a complex sodium aluminium silicate desilication product. The patent states that crystallization of desilication product (DSP) causes the dissolved silica to preferentially precipitate on the DSP particles, rather than on other surfaces such as heat exchange surfaces. The turbulence of the slurry in the digestion system can also act to maintain clean heat exchange surfaces. The DSP is insoluble and allows the slurry to pass to the digestion stage without scaling of heat exchange surfaces occurring. After digestion, the DSP is removed in the red mud residues.
A paper by Eremin, from the USSR Institute of Mining, Leningrad, entitled “Beneficiation of Low Grade Bauxite by Hydrometallurgical Methods” (in: Proc. Conference; Alumina Production until 2000, Tihany, Hungary, Oct. 6-9, 1981, p. 135-142.) discloses studies into the dissolution of silica components from bauxite. Following these studies, the paper concluded that bauxite desilication should be carried out at approximately 80 to 90° C. at a high liquid to solids ratio and with medium caustic concentrations (100 to 150 g/l Na
2
O, which corresponds to 170 to 260 g/l, calculated as Na
2
CO
3
). This paper makes reference to a treatment step in which a portion of the reactive silica is dissolved before Bayer process digestion. The reactive silica which enters solution is subsequently precipitated to produce a separable aluminosilicate material. However, this paper highlighted a major limitation in carrying out the process, since stable silica levels in solution never exceeded those expected in recycled Bayer liquors by more than about 2.5 gpL, limiting the effectiveness of silica dissolution at realis
Crisp Anthony John
Hollitt Michael John
Rodda Darren Paul
Roe Gerard Marcus
Staker Warren Scott
Bos Steven
Comalco Aluminum Limited
Dennison. Scheiner, Schultz & Wakeman
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